Study On Titania Based Ceramic Heating Coatings

The effect of processing parameters for TiO₂ based coatings to microstructures and electrifying heating properties of the coatings during plasma spraying was investigated based on the design of coatings components system and parameters optimization of coatings processing. The TiO₂ based ceramic heating coatings with different contents of Al₂O₃ were prepared by atmospheric plasma spraying using the optimized processing parameters. The relationship between Al₂O₃ contents and microstructures and electrifying heating properties of the coatings was established, the long-term electrifying heating properties and the conduction mechanism of TiO₂ based coatings were studied in detail. Furthermore, the microstructures evolution and the electrifying heating properties of TiO₂ based coatings during heat treatment at different temperatures were discussed. The major results are summarized as follows:The TiO₂ based ceramic heating coatings were prepared by atmospheric plasma spraying technology, and the porosity of ceramic coatings decreases with increased spraying current. The deoxidization phase Magneli appeared by the reduction of H₂ during plasma spraying. Ti₂O₃ phase further appeared with the increasing of H₂ flow. The electrical resistivity of the coatings decrease with the increasing spraying current and H₂ flow. Spraying distance affects the microstructure of coatings, thereby affecting the electrifying heating properties of coatings. The optimization plasma spraying parameters of coatings are: spraying current 550A, H₂ flow 6L/min, and spraying distance of 100mm.The heating temperature of TiO₂ based ceramic coatings increases with the increased Al₂O₃. The coating with 40% Al₂O₃ shows the highest heating temperature (490℃). The ceramic heating coatings can operate reliably at 330℃except TA5 coating.The presence of deoxidization phase Magneli in the coatings is the main factor of conducting. The Magneli phase can be oxidized during heat treatment of TiO₂ based coatings and makes the electrical resistivity of coating increasing. The phase transition completely from anatase to rutile of TiO₂ can be seen at 650℃. The TiO₂ based coatings occurred embrittlement to failure at 750℃. Therefore, the limit temperature of TiO₂ based coating application should be below phase transition temperature of 550℃.The electrifying heating test operating over long time periods shows that TiO₂ based coating can operate over 46 hours at high temperature (400℃) and over 170 hours at low temperature (190℃), both are better than TiO₂ coating. The addition of Al₂O₃ on TiO₂ based ceramic coatings improves high temperature resistance of coatings; and increases the electrifying heating temperature.The failure analysis of TiO₂ based heating coatings shows that the main reasons for coating failure are: the coating may occur arc cracks due to the high current; the coating may cause short circuit to be burned due to the local hot spot; the coating failure due to long-term thermal cycle makes the coating phase transition.